Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. Global climate models may be overstating the warming properties of black carbon particles, according to new research led by the University of California, Davis. The study will be published online Friday (Aug. 31) in the journal Science. Black carbon is a climate forcing agent formed through the incomplete combustion of fossil fuels, biofuel, and biomass, and is emitted in both anthropogenic and naturally occurring soot. It consists of pure carbon in several linked forms. Black carbon warms the Earth by absorbing heat in the atmosphere and by reducing albedo, the ability to reflect sunlight, when deposited on snow and ice.

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"Our results don’t change the fact that things are going to warm — just to what degree, literally," said Christopher Cappa, a UC Davis civil and environmental engineering professor. "Our findings should result in more accurate predictions."

A component of soot, black carbon is created through combustion — when fuels such as oil, coal and wood are burned. Because black carbon absorbs sunlight, it can warm the atmosphere. Reducing black carbon in the atmosphere has been a target for near-term climate mitigation. Earlier studies have suggested black carbon’s warming impacts are second only to carbon dioxide.

Approximately 20% of black carbon is estimated as emitted from burning biofuels, 40% from fossil fuels, and 40% from open biomass burning. Today, the majority of black carbon emissions are from developing countries and this trend is expected to increase. The largest sources of black carbon are Asia, Latin America, and Africa. China and India together account for 25-35% of global black carbon emissions.

Unlike greenhouse gases, which can live in the atmosphere for centuries, black carbon particles have lifespans of only one to two weeks, making it more difficult to quantify their impacts on a global scale through direct measurements. So scientists have had to rely more heavily on mathematical models to understand black carbon particles’ impacts on climate change.

Climate models regarding black carbon have been based on theories and laboratory experiments showing that as other chemicals are condensed onto black carbon particles, the warming properties of these particles increase.

In the new study, an international team of researchers left the lab and went into the field, where they used direct measurements to establish to what extent laboratory experiments translated to the real atmosphere.

The researchers found that the chemical and physical changes that occurred in the field had less impact on black carbon’s warming ability than lab experiments and models had forecast. Atmospheric changes to the black carbon particles increased light absorption by about 6 percent, rather than the 100 percent increase suggested by previous studies.

"This study has implications for thinking about the direct impact of black carbon on Earth’s energy budget," said Cappa.

In their new study, Cappa and his colleagues collected data during two field studies in 2010: CalNex 2010 and the Carbonaceous Aerosols and Radiative Effects Study (CARES). For CalNex, Cappa boarded the research vessel Atlantis and sailed along the California coast from San Diego to San Francisco. The team measured the light absorption by black carbon particles, the extent to which black carbon particles mixed with other chemical components in the urban atmosphere, and how black carbon particles changed over time. During CARES, measurements were made at a ground site in Sacramento.